The present invention relates to the art of surface protection treatments, more specifically, it relates to water borne protective coatings obtained from a cured admixture of a Mannich base of a hydroxystyrene homopolymer or copolymer and a blocked isocyanate.
The desire for applying protective coatings to metal and other surfaces for the purpose of enhancing resistance to corrosion is well known in the art. Indeed, a plethora of coating compositions directed to this end are well documented in the published literature and many are commercially available as well. Typical protective coatings may include pigments suspended in a vehicle. The vehicle consists primarily of a resinous binder dissolved in solvents or dispersed in water together with small quantities of driers, plasticizers, and stabilizers as required by the intended end use. As the film dries the vehicle changes from a liquid to a solid film either by evaporation of water or solvents, oxidation or polymerization through the application of heat, acceleration by a catalyst or a combination of reactive components. To this end protective coatings include oleoresinous paints or drying oils; oil based paints; alkyd, phenolic, epoxy, chlorinated rubber silicone, vinyl, acrylic, nitrocellulose, polyester and poly-urethane paints and varnishes. The selection of coating depends on its intended application and the degree of protection required for the ambient conditions.
Polyurethane coatings ar formed through the reaction of a polyisocyanate with a substance containing reactive hydrogen such as a hydroxyl bearing polyester or polyether. Polyurethane coatings include formulations which are one-package moisture-cured, two- component polyol-cured and oil modified.
In two-component polyol-cured formulations, the polyisocyanate fraction usually is partially prepolymerized with a portion of the polyol; otherwise the very short cure time after mixing would make this coating very difficult to apply.
The polyol fraction, usually consisting of various hydroxyl bearing polyesters, polyethers, or blends of these and contains the pigment for pigmented coatings.
In one-package moisture-cured formulations, the coatings consist of solutions of the diisocyanate fraction which have been prepolymerized with as much of the polyol as possible without causing gelling. The final cure is derived from moisture adsorbed from the atmosphere after application.
Because the prepolymer usually is a viscous material, solvents are employed. This is a potentially dangerous formulation since any unreacted diisocyanate will escape with the evaporating solvent and many isocyanate exhibit serious toxicity properties.
In oil-modified formulations, in the usual one-package formulation, the diisocyanate is completely reacted with polyalcohol esters of drying oils.
The present invention employs a polyisocyanate which has been essentially completely blocked in admixture with hydroxystyrene homopolymers or copolymers in a suitable medium which is preferably water. Cationic electrocoatings may be prepared by dispersing the hydroxystyrene (co)polymer-blocked polyisocyanate mixture in water or water/co-solvent media. Water is preferably employed so that cationic electrocoatings may be prepared from a dispersion of the solids in water.
The films are thermoset, usually in the presence of a catalyst. It is hypothesized that the polyisocyanate is unblocked and then reacts with the active hydrogen members of the Mannich base to form a cohesive, cured film.